1. Field of the Invention
The present invention relates to a secondary battery and a method of forming the same, and more particularly to a plastic molding type secondary battery in which a gap between a bare cell and a protective circuit board is filled with a plastic molding, and a molding method thereof.
2. Description of the Prior Art
As is generally known in the art, secondary batteries are rechargeable and can be made in a compact form with a large capacity. Thus, secondary batteries have been recently broadly researched and developed. Typical examples of such secondary batteries include nickel metal-hydride (Ni-MH) batteries, lithium (Li) batteries and lithium-ion (Li-ion) batteries.
In such secondary batteries, most bare cells are formed by inserting an electrode assembly including a positive electrode, a negative electrode and a separator into a can formed from a metal such as, for example, aluminum, coupling a cap assembly to the can, injecting an electrolyte into the can, and then sealing the can.
However, a battery is an energy source and the battery has the potential to discharge a large amount of energy. In the case of a secondary battery, a large amount of energy is stored in the battery when it is charged. Also, in order to charge the secondary battery, an external energy source is needed for supplying the energy to be stored in the battery. When an internal short circuit or other disorder of the secondary battery is generated during the above described process or state, the energy stored in the battery may be discharged in a short time, thereby causing safety problems such as fire, explosion, or the like.
Accordingly, in general, a secondary battery is equipped with various kinds of safety devices for preventing fire or explosion caused by disorder of the battery itself in a charged state or during the charging process of the battery. Among these safety devices, a protective circuit board is generally connected to a positive terminal and a negative terminal of a bare cell by a conductive structure, a so-called lead plate. Such a protective circuit board may further include a Positive Temperature Coefficient (PTC) device or a bimetal device. In this case, the protective circuit board can break electric current, for example, when a battery is heated to a high temperature or when battery voltage rapidly increases due to, for example, overcharging/overdischarging, thereby preventing dangers such as explosion and firing of the battery.
A secondary battery including a bare cell coupled with a protective circuit board is enclosed in a separate casing to provide a finished secondary battery. In some cases, the gap between a bare cell and a protective circuit board connected by welding, is filled with a plastic molding to provide a plastic molding type secondary battery. For purposes of downsizing a plastic molding type secondary battery, an external I/O (Input/Output) terminal for making an electric connection between the battery and an external circuit may be formed on one surface of the protective circuit board. This allows the I/O terminal to be exteriorly exposed without the use of, for example, a lead wire.
FIG. 1 is a schematic exploded perspective view showing a conventional pack-type lithium-ion battery before coupling with a plastic molding. FIG. 2 is a sectional view showing a conventional pack-type lithium-ion battery in which a bare cell and a lead plate of a protective circuit board are welded to each other and mounted in a mold.
Referring to FIGS. 1 and 2, a protective circuit board 30 is disposed parallel with a surface, on which electrode terminals 11, 12 of a bare cell of a pack-type battery are formed. Additionally, as shown in FIG. 2, a gap 47 exists between the bare cell 10 and the protective circuit board 30 and may be filled with a plastic molding. When the plastic molding is filled, it may cover the back surface opposite the surface of the protective circuit board facing the bare cell. However, an external I/O terminal 32 formed on the back surface must be exposed to the exterior.
The bare cell 10 includes a positive terminal 11 and a negative terminal 12 on the surface facing to the protective circuit board 30. The positive terminal 11 may be, for example, a cap plate or a metal plate bonded to a cap plate. The negative terminal 12 is a terminal protruding vertically from a cap plate, and is electrically isolated from the cap plate 13 by a peripheral insulator gasket.
The protective circuit board 30 includes a panel formed of a resin, on which a circuit is disposed, and the external I/O terminal 32, formed on the outer surface thereof. The protective circuit board 30 has dimensions and a shape which are substantially the same as those of the surface (cap plate surface) of the bare cell facing thereto.
The back surface of the protective circuit board 30 opposite the surface on which multiple external I/O terminals 32 are formed, i.e., the internal surface of the protective circuit board, is equipped with a circuit section 35 and connection terminals 36, 37. The circuit section 35 includes, for example, a protective circuit for protecting a battery from overcharging/overdischarging during charging/discharging of the battery. The circuit section 35 and each external I/O terminal 32 are electrically connected to each other by a conductive structure passing through the protective circuit board 30.
Lead plates 41, 42 and an insulating plate 43 are disposed between the bare cell 10 and the protective circuit board 30. The lead plates 41, 42, generally formed of nickel, are used to make an electric connection between positive terminal 11 and negative terminal 12 and its respective connection terminal 36, 37 of the protective circuit board 30. Also, they may have an “L”-shaped form or a planar structure. By connecting each terminal 36, 37 of the lead plates 41,42 through, for example, welding, it is possible to make electric connections, but such connections may not ensure mechanical integrity of the entire structure. In other words, even if the lead plates 41, 42 are welded with the connection terminals 36, 37, the protective circuit board may not be firmly fixed to the bare cell.
Further, as shown in FIG. 2, when the bare cell 10 welded to the protective circuit board 30 is mounted into a mold 100, it is necessary to protect the surface of the external I/O terminal 32 from being covered with a plastic molding in order to allow an electric connection to be made with an external circuit. If the external I/O terminal 32 is formed to protrude from the surface of the protective circuit board, it is possible for the surface of the terminal 32 to be in direct contact with the inner surface of the mold 100, as shown in FIG. 2. In addition to this method, it is also possible to form the mold 100 to take a protruded inside shape sufficient to be in contact with the external I/O terminal 32 of the protective circuit board for the purpose of protecting the surface of the terminal 32.
In order to protect the surface of the external I/O terminal 32, it is possible for the surface to be in contact with the inner surface of the mold, from the starting point of mounting the protective circuit board 30 in the mold. However, in this process, there is a great potential to generate a gap between the inner surface of the mold and the surface of the external I/O terminal 32 of the protective circuit board. To solve this problem, another method may be used. Specifically, the protective circuit board 30 and the bare cell are mounted in the mold 100, wherein the inner surface of the mold facing the surface of the terminal 32 is movable, and then that surface is moved toward the surface of the terminal 32.
More particularly referring to FIG. 3, it is possible to place the surface of the mold 100 in contact with the external I/O terminal 32 by moving a part of the surface of the mold toward the surface of the terminal so as to form an opening in the mold at a location corresponding to the external I/O terminal, inserting a post-type core 210 into the opening, and advancing the core 210 to protrude into the inner surface of the mold 100.
When a core 210 is used in the mold, the plastic molding process may be performed as follows. First, a protective circuit board 30 connected to a bare cell 10 is mounted in a mold 100 adapted to receive the protective circuit board 30 in a process of forming a secondary battery. Next, the core 210 is moved toward the protective circuit board 30 through an opening of the mold 100, so that the front-end 212 of the core 210 is in contact with the surface of the external I/O terminal 32 of the protective circuit board 30. Finally, a liquid plastic resin is injected into the mold 100, and then the plastic resin is solidified.
However, in the above-described process, when the core 210 is moved, the protective circuit board 30 is fixed to the bare cell 10 only by a connection between each thin lead plate 41,42 and each connection terminal 36, 37 which tends to push the protective circuit board irregularly toward the bare cell 10. In this case, as shown in FIG. 3, the protective circuit board 30 may be oriented away from its correct position, and thus a fine wedge-shaped gap 214 may be generated between the front-end of the core 210 and the surface of the external I/O terminal 32 of the protective circuit board. If the gap 214 is filled with a plastic molding 20, a problem, so-called a “flash defect,” may be generated. A flash defect may occur when a part of the surface of the external I/O terminal 32 is coated with a plastic film 21, as shown in FIG. 4. Additionally, due to the incorrect position of the protective circuit board, other factors responsible for quality below defined standards may be generated.
In order to prevent the protective circuit board from being pushed irregularly toward the bare cell while the core is moved, a method of disposing a support structure made of the same plastic as the molding between the bare cell and the protective circuit board may be used to mount the protective circuit board coupled to the bare cell by lead plates. However, such a support structure may cause another problem in that it may disturb efficient injection of a plastic resin in the step of pouring the plastic into the mold. Accordingly, there is a need for a method and a device to solve the problems as described above.